1. Field of the Invention
This invention relates generally to communications, and more particularly, to a multicarrier communications system and method that are able to controllably change an overhead channel data transmission rate.
2. Brief Description of Related Prior Art
The public switched telephone network (PSTN) provides the most widely available form of electronic communication for most individuals and businesses. Because of its ready availability and the substantial cost of providing alternative facilities, it is increasingly being called upon to accommodate the expanding demands for transmission of substantial amounts of data at high rates. Structured originally to provide voice communication with its consequent narrow bandwidth requirements, the PSTN increasingly relies on digital systems to meet the service demand.
A major limiting factor in the ability to implement high rate digital transmission has been the subscriber loop between the telephone central office (CO) and the premises of the subscriber. This loop most commonly comprises a single pair of twisted wires which are well suited to carrying low-frequency voice communications for which a bandwidth of 0-4 kHz is quite adequate, but which do not readily accommodate broadband communications (i.e., bandwidths on the order of hundreds of kilohertz or more) without adopting new techniques for communication.
One approach to this problem has been the development of discrete multitone digital subscriber line (DMT DSL) technology and its variant, discrete wavelet multitone digital subscriber line (DWMT DSL) technology. These and other forms of discrete multitone digital subscriber line technology (such as ADSL, HDSL, etc.) will commonly be referred to hereinafter generically as xe2x80x9cDSL technologyxe2x80x9d or frequently simply as xe2x80x9cDSLxe2x80x9d. The operation of discrete multitone systems, and their application to DSL technology, is discussed more fully in xe2x80x9cMulticarrier Modulation For Data Transmission: An Idea Whose Time Has Come,xe2x80x9d IEEE Communications Magazine, May, 1990, pp. 5-14.
In DSL technology, communications over the local subscriber loop between the central office and the subscriber premises is accomplished by modulating the data to be transmitted onto a multiplicity of discrete frequency carriers which are summed together and then transmitted over the subscriber loop. Individually, the carriers form discrete, non-overlapping communication subchannels of limited bandwidth; collectively, they form what is effectively a broadband communications channel. At the receiver end, the carriers are demodulated and the data recovered from them.
The data symbols that are transmitted over each subchannel carry a number of bits that may vary from subchannel to subchannel, dependent on the signal-to-noise ratio (SNR) of the subchannel. The number of bits that can accommodated under specified communication conditions is known as the xe2x80x9cbit allocationxe2x80x9d of the subchannel, and calculated for each subchannel in a known manner as a function of the measured SNR of the subchannel and the bit error rate associated with it.
The SNR of the respective subchannels is determined by transmitting a reference signal over the various subchannels and measuring the SNR""s of the received signals. The loading information is typically calculated at the receiving or xe2x80x9clocalxe2x80x9d end of the subscriber line (e.g., at the subscriber premises, in the case of transmission from the central telephone office to the subscriber, and at the central office in the case of transmission from the subscriber premises to the central office) and is communicated to the other (transmitting or xe2x80x9cremotexe2x80x9d) end so that each transmitter-receiver pair in communication with each other uses the same information for communication. The bit allocation information is stored at both ends of the communication pair link for use in defining the number of bits to be used on the respective subchannels in transmitting data to a particular receiver. Other subchannel parameters such as subchannel gains, time and frequency domain equalizer coefficients, and other characteristics may also be stored to aid in defining the subchannel.
Information may, of course, be transmitted in either direction over the subscriber line. For many applications, such as the delivery of video, internet services, etc. to a subscriber, the required bandwidth from central office to subscriber is many times that of the required bandwidth from subscriber to central office. One recently developed service providing such a capability is based on discrete multitone asymmetric digital subscriber line (DMT ADSL) technology. In one form of this service, up to two hundred and fifty six subchannels, each of 4312.5 Hz bandwidth, are devoted to downstream (from central office to subscriber premises) communications, while up to thirty two subchannels, each also of 4312.5 Hz bandwidth, provide upstream (from subscriber premises to central office) communications. Communication is by way of xe2x80x9cframesxe2x80x9d of data and control information. In a presently-used form of ADSL communications, sixty eight data frames and one synchronization frame form a xe2x80x9csuperframexe2x80x9d that is repeated throughout the transmission. The data frames carry the data that is to be transmitted; the synchronization or xe2x80x9csyncxe2x80x9d frame provides a known bit sequence that is used to synchronize the transmitting and receiving modems and that also facilitates determination of transmission subchannel characteristics such as signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d), among others.
A DMT standard has been set for DSL transmission by the ANSI Standards body for full-rate ADSL in the publication xe2x80x9cT1E1.4/97-007R6 Interface between net-work and customer installation asymmetric digital subscriber line (ADSL) metallic interface, xe2x80x9d published Sep. 26, 1997xe2x80x94referred to hereinafter as xe2x80x9cT1.413 Issue 2xe2x80x9d. This standard has also been recommended as the standard modulation technique to be used for splitterless DSL operation by the Universal ADSL Working Group (UAWG) (See: xe2x80x9cUniversal ADSL Framework Document TG/98-10R1.0,xe2x80x9d published by the UAWG on Apr. 22, 1998, and referred to hereinafter as xe2x80x9cUADSL specificationxe2x80x9d). A variation of this standardized DMT technique is also expected to be approved as a standard, termed G.Lite,xe2x80x9d by the International Telecommunications Union. According to these standardized DMT techniques, hundreds of 4.3125 kiloHertz (kHz) subchannels are used for DSL transmissions between a telephone company central office (CO) and remote terminal (RT) or customer premises (at a home or business). Data are transmitted in both the downstream direction (from the CO to the RT) and the upstream direction (from the RT to the CO). According to these standards, the aggregate bandwidth (i.e. the sum of the bandwidths used in both upstream and downstream transmissions) of a full rate ADSL system is over 1 megaHertz (MHz), while that of G.Lite is over 500 kHz.
A superframe is 17 milliseconds in duration. A frame is effectively 250 micro-seconds in duration (or conversely, the frame rate is approximately 4 kHz) and is made up of a collection of bytes (with one byte corresponding to 8 bits).
After one DSL modem has initialized and established an active communication session with another DSL modem, the modems enter a steady state or information transmission mode. In this mode, data are transported in the upstream direction and the downstream direction at data rates that were determined during the initialization process in which the session was established. During steady state mode, each frame of data transmitted/received by the modem is made up of an overhead section and a payload section. The overhead section carries information that is used to manage the communications between the two communicating DSL modems, while the payload section contains the actual (e.g., user) data to be communicated between the modems. In DSL communications that conform to the DMT communications standards whose specifications are referenced above, the first byte of each frame of data is designated as an overhead byte. The overhead section may comprise cyclic redundancy check (CRC) data, indicator bit (IB) data, embedded operations channel (EOC) data and ADSL over-head channel (AOC) data. Cyclic redundancy data are used to check the integrity of the communications link between the two DSL modems. Indicator bit data are used to indicate certain communications error conditions that may occur during the communications session. EOC and AOC data provide information concerning the status of the communications session. The format and information provided by these portions of overhead data are described in detail in T1.413 Issue 2. (See, e.g., Sections 6.4.1.3, 8.1, 10.1 and Table 3 of the T1.413 Issue 2)
As described in T1.413 Issue 2, data can be transported between the communicating modems during a given DSL communications session either with data interleaving or without data interleaving. If data interleaving is employed, the transported data are channeled through an xe2x80x9cInterleave Bufferxe2x80x9d. Conversely, if transported data are not interleaved, the data may be channeled through a xe2x80x9cFast Bufferxe2x80x9d. As noted previously, the first byte in each frame is an overhead data byte. When data interleaving is employed, this overhead byte is termed a xe2x80x9csync bytexe2x80x9d; however, when interleaving is not employed, the overhead byte may be termed a xe2x80x9cfast byte.xe2x80x9d
Table 1 below is taken from Table 7 of the T1.413 Issue 2, and illustrates how overhead data may be distributed in frames transmitted during a conventional DSL communications session, wherein a xe2x80x9creduced overhead modexe2x80x9d of operation is employed. As is described in detail in Section 6.4.4.2 of the T1.413 Issue 2, in the xe2x80x9creduced overhead modexe2x80x9d of operation, the sync or fast bytes are xe2x80x9cmerged.xe2x80x9d
As depicted in Table 1 above, the first overhead byte in the first frame is used to transport CRC data. The first byte in the second frame is used to transport the first 8 indicator bits. The first byte in the 34th frame is used to transport the eighth through the fifteenth indicator bits. The first byte in the 35th frame is used to transport the sixteenth through the twenty-third indicator bits. The first byte in all the remaining frames alternates between either EOC data or AOC data. However, in this conventional scheme, when actual EOC or AOC data are not available for transport, which can often occur when according to the scheme, EOC or AOC data are to be comprised in a frame, pre-determined dummy bytes are used instead of unavailable actual EOC or AOC data.
Unfortunately, since one byte out of each frame in each superframe during conventional DSL communications is dedicated to overhead data, the corresponding overhead data rate is invariably fixed at 32 kbps, and is not changed when either the payload data transmission rate changes or when no actual EOC or AOC data are available for inclusion in the frame. Further, some telephone lines used in DSL communications are of such poor quality that the maximum possible DSL data transmission rate using such lines may not exceed 128 kbps. Unfortunately, this means that when DSL communications are carried out over such poor quality lines, an undesirably large proportion (e.g., up to twenty-five percent) of the DSL communications system""s throughput may be used to transmit overhead data. At any given time during a given communications session, the total communications bandwidth is constant. Thus, since the total data communications transmission rate either upstream or downstream, as the case may be, at any given time during a DSL communications session, is constant, this means that communications bandwidth that otherwise would be available to transmit payload data is unnecessarily consumed in transmitting overhead data.
In general, it is an object of the present invention to provide a multicarrier communications system and method that overcome the aforesaid and/or other disadvantages and drawbacks of the prior art, and more specifically, to provide such a system and method wherein the overhead data transmission rate during a communications session may be changed and/or selected.
Accordingly, a multicarrier communications system and method are provided that are able to overcome the aforesaid and other disadvantages and drawbacks of the prior art. In the system and method of the present invention, the overhead data transmission rate may be changed and/or selected. More specifically, this rate may be selected during an initial negotiation process and/or during a steady state mode of operation.
In one embodiment, the system of the present invention may comprise two DMT DSL modems, one located at a customer premises and another located at a telephone central office, connected by a conventional POTS line through which the modems communicate by transmitting and receiving discrete frames and superframes of data. Within each superframe are 68 data frames and a synchronization symbol. Within each frame is a number of bytes that are allocated to payload and overhead data. The allocation of the bytes to either overhead or payload data is flexible (i.e., changeable and/or selectable). Whereas in the prior art, the first byte in each frame is dedicated to overhead data regardless of whether there is a need to transport overhead data or not, in this embodiment of the present invention, the overhead data transmission rate is determined during start-up and can be modified during steady state mode. Due to the construction of frames in DSL systems, decreasing the overhead data transmission rate during steady state mode results in a higher payload data transmission rate, while conversely, increasing the overhead data transmission rate during steady state mode results in a lower payload data transmission rate.
Flexible Overhead Allocation
As noted previously, in conventional DSL systems, one byte per frame is dedicated to overhead data. In the improved system of this embodiment of the present invention, both the number of bytes and the frame(s) comprising overhead data may be selected. By selecting the number of frames that comprise overhead data, and the number of bytes allocated to overhead data in those frames, the amount of throughput that is dedicated to overhead data can modified. This is a marked departure from conventional DSL systems wherein the amount of throughput that is dedicated to overhead data is unchangeably fixed at 32 kbps.
Similarly, in this embodiment of the present invention, it is possible to select which of the superframes are to carry overhead data-containing frames. This introduces another degree of freedom in allocating the overhead and payload data transmission rates.
Also advantageously, since the overhead data transmission rate is selectable in this embodiment of the present invention, it is possible to select that rate based upon the relative priorities that are desired to be given to transmission of payload and overhead data, and/or whether there is a need to have a high overhead data transmission rate because a given application requires it (e.g., if compressed voice data is to be transported via an overhead data channel).
Control commands may be exchanged between the modems during their initial negotiation or handshake phase that may govern how many and which of the frames and/or superframes may contain overhead data, and the number of bytes of such data in the effected frames. These control commands may comprise respective messages whose receipt by a modem during initial negotiation may cause the modem to select from a plurality of sets of parameters, a respective set of parameters that will govern how many and which frames and/or superframes will contain overhead data, the number of bytes of such data in the effected frames, etc. during the communications session between the modems. These sets of parameters may be stored in table form in each of the modems, and may designate which the particular bytes, frame(s), and superframe(s) are to be dedicated to overhead data.
Dynamic Overhead Data Throughput Allocation
In addition to permitting the amount of throughput devoted to overhead data to be selectable, this embodiment may also permit dynamic adjustment of that throughput during steady state operation.
For example, after establishing the overhead data transmission rate during startup negotiation, a new messaging process may allow renegotiating of this data transmission rate during steady state operation, as necessary. For example, a 4 kbps overhead data rate may be initially negotiated during startup, and thereafter, if a large EOC data transfer is required, a new overhead channel data transmission rate (for example 32 kbps) could be negotiated, to permit the overhead data to be quickly transmitted. Upon completion of that data transfer, the overhead data transmission rate may then be renegotiated, as appropriate.
The dynamic renegotiations of the overhead data transmission rate during steady state operations may be effected by exchange of control commands between the central office and customer premises"" modems, in a manner similar to that used to initially negotiate that rate. These control commands may be exchanged via the overhead channels. Similarly, the commands exchanged may comprise respective messages whose receipt by a modem during renegotiations of the overhead data transfer rate may cause the modem to select from a plurality of sets of parameters, a respective set of parameters that will govern how many and which frames and/or superframes will contain overhead data, the number of bytes of such data in the effected frames, etc. during further communications between the modems. These sets of parameters may be stored in table form in each of the modems and may designate which the particular bytes, frame(s), and superframe(s) are to be dedicated to overhead data. The messages may comprise one or more tones, or may comprise use of a predetermined protocol over an overhead channel, that identify the particular parameter set.
Once the change in overhead data transmission rate has been renegotiated, in order to effectuate further exchange of overhead data, the modems involved in the renegotiation must synchronize their transmission/reception of overhead data in accordance with newly negotiated rate. In accordance with this embodiment of the present invention, there are several alternative techniques by which this synchronization may be accomplished. In the first such technique, the central office modem may keep an internal count of the frames/superframes that have been transmitted from that modem to is the customer premises modem with which it has been communicating, and the customer premises modem may likewise keep an internal count of the frames/superframes that it has received from the central office modem. A message may be passed from one of the modems to the other modem that contains a frame/superframe count value at which the two modems are to adjust their overhead data transmit/receive rates in accordance with the newly negotiated rate. Each modem then adjusts its overhead data transmit/receive rate when its respective internal frame/superframe count reaches that value.
Alternatively, one of the modems may transmit to the other modem a flag message indicating that, when the other modem transmits to the modem sending the flag message a specified subsequent superframe (e.g., the next superframe), the overhead data transmission/reception rates are to be adjusted in accordance with the newly negotiated rate. Upon transmission of that specified superframe, the modem that transmitted the superframe adjusts to the newly negotiated rate; likewise, upon receipt of the specified superframe, the modem receiving that superframe adjusts to the newly negotiated rate.
Of course, it will be appreciated that a request to renegotiate the overhead data transmission rate can originate from either the modem at the central office or from the modem at the customer site. Further, that request may be initiated by either the transmit block or the receive block in the modem initiating the request.
These and other features and advantages of the present invention will become apparent as the following Detailed Description proceeds and upon reference to the FIGS. of the Drawings, wherein: